Pathologic synaptic reorganization of mossy fiber axons of the dentate granule cells, identified in human temporal lobe epilepsy and in many animal models, may contribute to the formation of abnormal recurrent excitatory synapses whereby the granule cells excite themselves or their neighbors and may enhance development of spontaneous seizures. The objective of this proposal is to identify genes responsible for morphologic alteration of neural circuitry and develop an experimental system in mice combined with the power of genetics which will facilitate elucidating the molecular basis of mossy fiber sprouting similar to an in vitro model of mossy fiber sprouting in rat hippocampal explants. Recent work from my preceptor's laboratory, implicating one possible molecular determinant (c-fos) of mossy fiber sprouting, demonstrates that -/- mutant c-fos mice exhibit a striking attenuation of kindling induced mossy fiber sprouting. Specific aims are threefold: 1) Implement a model of status epilepticus-induced mossy fiber sprouting in mouse hippocampal explant culture; 2) Is mossy fiber sprouting attenuated in a gene dosage dependent manner in explants isolated from +/+, +/-, and -/- mutant c-fos mice; and 3) rescue defective mossy fiber sprouting in explants prepared from -/- c- fos mice by adenoviral c-fos gene transfection. Timm staining, neurobiotin fills, viral transgene transfection, in situ hybridization, and fluorescent microscopy will be used to characterize the role of c-fos in mossy fiber sprouting. Both c-fos alleles will be necessary but not sufficient for full expression of mossy fiber sprouting following repeated seizures. These studies should provide a framework to investigate a variety of candidate genes and suggest novel pharmacologic interventions aimed at regulating mossy fiber sprouting.